FR3047309A1 - METHOD AND DEVICE FOR MONITORING THE MECHANICAL BEHAVIOR OF AN UNDERWATER CONDUCT OF TRANSPORTING PRESSURIZED FLUIDS - Google Patents

Abstract

The invention relates to a method for monitoring the mechanical behavior of a subsea pipeline (2) for transporting pressurized fluids by assembling unitary pipe elements (4), the method comprising a step of mechanical calibration consisting of using a measuring cable equipped with an optical fiber sensor, to measure the deformations undergone by each unit pipe element when it is subjected to the earth at different mechanical stresses of predetermined directions and intensities, and from these measurements, to establish the mechanical signature of each unit driving element, and a monitoring step consisting, using a measuring cable (18) provided with a fiber optic sensor using the Brillouin backscattering and helically positioned at a constant pitch (p) on each unitary pipe element with alternating pitch direction of the helix for two adjacent pipe units cent, to recover the variations of the optical signal injected into the sensors when the pipe is in use, and, from the variations of the signal and the mechanical signature of each unitary pipe element, to determine the deformations undergone by each unit element of conduct.

Description

Title of the invention

Method and device for monitoring the mechanical behavior of an underwater pipe for conveying fluids under pressure

Background of the invention

The present invention relates to the general field of submarine fluid transport pipes resting at the bottom of the sea or ensuring the bottom-surface connection for the transfer of hydrocarbons, for example oil and gas, from production wells. submarines.

Underwater fluid lines are commonly used in offshore hydrocarbon production. Thus, in an offshore production field, several wells are generally exploited that can be separated from each other by several kilometers or even tens of kilometers. The fluids from the various wells must be collected by pipelines placed at the bottom of the sea and transferred by bottom-surface connection pipes from an underwater pipe resting at the bottom of the sea to a surface installation which will collect them, for example at a ship or a collection point located on the coast.

There are different types of underwater pipes used for the transport of fluids. The invention is more particularly concerned with coaxial pipes of the "Pipe In Pipe" or PIP type, that is "pipe-in-pipe" type, in which an internal steel tube transports the fluids and a steel tube. coaxial outer to the previous, also called "external envelope", is in contact with the environment, that is to say with water. Generally, these coaxial pipes are assembled on the ground in elements of unit length (one speaks of double, triple or quadruple joints, hereinafter indifferently denominated "quad-joints" for quadruple sections of tube), of the order of 10 to 100m according to the carrying capacity of the installation system. These quad-joints are then transported at sea on a laying ship.

During the installation, the quad-joints are connected to each other on board the ship and as and when they are laid at sea. Laying can be done by means of a J-shaped positioning tower on the laying ship. With the J-laying, the underwater pipe is typically lowered from the laying vessel practically vertical (between + 30 ° and -10 ° relative to the vertical). Laying J is a simple catenary installation in which the almost vertical inclination of the pipe decreases as it moves downwards to marry the slope of the seabed.

Given their specificity, submarine fluid transport lines are designed to achieve a high level of thermal performance and specific versions have been developed to respond more appropriately to deep seabed, ie to withstand the pressure of the sea floor. In fact, since the water pressure is approximately 0.1 MPa, ie approximately 1 bar per 10m depth, the pressure to which the underwater pipes must withstand is then approximately 10 MPa. about 100 bar for 1000m depth and about 30 MPa, about 300 bar for 3000m depth.

In addition, underwater pipelines are subjected to significant mechanical stresses, both during their installation at the seabed via the J-laying tower, which causes significant deformations (especially bending) of each quad-joint of the pipe, but also during the production phase (self-stress and stresses due to external stresses). Indeed, when the submarine pipe is installed at the bottom of the sea and the network is in production phase, the inner pipe of the quad-joints of the pipe is subjected to the high pressure of the fluids that it transports (this pressure may exceed 100 bar). In addition, it is common that the seabed on which the pipe rests moves, resulting in movements of the pipe, and therefore constraints on the latter.

However, the mechanical stresses experienced by the subsea pipes, and more particularly the internal pipes of PIP pipes, as well as the temperature to which they are subjected (the hydrocarbons coming from the subsea production wells come out at a temperature of order of 70 ° C), may cause significant damage or rupture, pipes.

Object and summary of the invention

There is therefore a need to be able to monitor the mechanical behavior of an underwater pipe for transporting fluids, both during its installation and in the production phase.

According to the invention, this requirement is achieved by means of a method for monitoring the mechanical behavior of an underwater pipe for conveying fluids under pressure, the underwater pipe being made by assembling a plurality of unitary elements. of pipes placed end to end, the method comprising: a mechanical calibration step consisting of at least one measuring cable provided with at least one fiber optic sensor using at least Brillouin backscattering and positioned along the entire length of the unitary pipe elements, to measure the deformations experienced by each unit pipe element when it is subjected to the earth at different mechanical stresses of predetermined directions and intensities, and, from these deformation measurements, to establish the mechanical signature of each unitary driving element; and a monitoring step consisting of a measuring cable provided with at least one fiber optic sensor using Brillouin backscattering and positioned in a constant pitch helix on each unitary conductor element with an alternation of the direction the pitch of the helix for two adjacent unitary conductor elements, recovering the variations of the optical signal injected into said optical fiber sensors when the pipe is in use, and, from these variations of the optical signal and the mechanical signature of each unitary driving element, to determine the deformations undergone by each unitary driving element

Correlatively, the subject of the invention is a device for monitoring the mechanical behavior of an underwater conduit for conveying fluids under pressure, the underwater pipe being made by assembling a plurality of unitary elements of pipes placed at the end of the pipe. the device comprising, for each unitary pipe element, at least one measuring cable provided with at least one optical fiber sensor using Brillouin backscattering and positioned in a constant pitch helix on said unitary driving element, two adjacent unitary conductors being provided with measuring cables whose optical fiber sensors are helically positioned with alternating pitch of the helix.

The monitoring method according to the invention is remarkable in particular that it uses mechanical signatures that are specific to each unit element (or quad-joint) of the underwater pipe to monitor the mechanical stresses undergone by the pipe. Prior to installation, during the calibration step, each unit pipe element is tested on the ground (in the laboratory or in the factory) and undergoes different mechanical stresses of predetermined directions and intensities. The deformations reflected on the unit driving element are measured by the fiber optic sensors.

During the actual monitoring step, the measurements are made by optical fiber sensors which are helically positioned around quad-joint with alternating pitch direction of the helix for two adjacent quad-joints. This particularly advantageous arrangement of optical fiber sensors (in particular in that it requires a reduced number of optical fiber sensors for monitoring multiple mechanical stresses) is made possible by starting from the hypothesis on the spatial homogeneity of certain stresses. mechanics undergone by the quad-joints of the underwater pipe. The optical fiber sensors thus positioned are used to determine, by means of the mechanical signatures of the unit elements of the pipe, the deformations experienced in service by each of the unit elements of pipe. From the knowledge of the mechanical behavior of each unit driving element, it is then possible to reconstruct the overall mechanical behavior of the underwater pipe, both during its installation and during the production phase.

The method and the monitoring device according to the invention also make it possible to know precisely the state of the underwater pipe at the connections between the different unit elements of pipe, which is not possible with a single line of measurements over the entire length of the pipe.

The method according to the invention is moreover particularly adapted to the J-laying technique of the submarine pipe during which no permanent deformation is induced to the pipe (it remains under 90% of the elastic limit of the driving).

Once laid, the pipe always remains in a linear domain (at less than 66% of the elastic limit), which allows a linearization of the perturbation phenomena undergone by the pipe (such as the deformations).

The mechanical signature of a unitary driving element may comprise a stiffness matrix whose values correspond to the deformation values experienced by the unitary driving element when it has been subjected to the earth at different mechanical stresses. In this case, the determination of the deformations undergone by each unitary driving element comprises the inversion of this stiffness matrix associated with the mechanical signature of each unitary driving element. Thus, the stiffness matrix represents the mechanical identity specific to each unitary driving element.

Preferably, the helix formed by the optical fiber sensor of each unit pipe element forms an angle with a longitudinal axis of the underwater pipe of 45 ° in absolute value.

The mechanical stresses of predetermined directions and amplitudes applied on the ground to each unitary pipe element may include one or more of the following stresses: torsional stress, tensile / compressive stress, flexural stress, and pressure stress.

The fiber optic sensor measurement cables may be positioned on the unitary conductor elements so as to extend substantially parallel to a longitudinal axis of said conduit.

Preferably, the optical fiber sensors of the measurement cables using Brillouin backscattering are monomode fibers.

In an application to the monitoring of a PIP pipe, each unit pipe element may comprise an inner tube mounted inside an outer tube coaxially with it, the fiber optic sensor measuring cables being positioned on the inner tube of said unitary pipe element.

The method may further comprise: an acoustic calibration step consisting, using a measuring cable provided with the fiber optic sensor using at least Rayleigh backscattering and positioned along the entire length of the unitary conductor elements, measuring the frequency variations of each unit pipe element when subjected to different acoustic stresses on the ground, and from these noise variations establishing an acoustic signature of each unit pipe element; and a step of monitoring the noise variations within the underwater pipe by recovering the frequency variations of the optical signal injected into the fiber optic sensors when the pipe is in use, and from these variations of frequencies and the acoustic signature of each unitary driving element, to determine the noise variations within each unitary driving element.

Thus, optical fiber sensors using Brillouin backscattering can not only provide information on the mechanical stresses experienced by the pipe, but also information on the variations of surrounding vibrating sources. Among these sources of surrounding vibrations, it is particularly possible to distinguish those generated by the unitary conduct element on which are positioned the optical fiber sensors and that, generated by the external environment (in this case the seabed and everything connected to it).

BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages of the present invention will emerge from the description given below, with reference to the accompanying drawings which illustrate embodiments having no limiting character. In the figures: - Figure 1 is a schematic view of an underwater pipe equipped with a monitoring device according to the invention; FIG. 2 is a diagrammatic view of an underwater pipe quad-joint to which various mechanical stresses are applied in order to establish its mechanical signature; and FIG. 3 is a perspective view showing the arrangement of the optical fiber sensors of the monitoring device of FIG. 1 on two adjacent quad-joints of the underwater pipe.

DETAILED DESCRIPTION OF THE INVENTION The invention applies to any submarine pipe resting at the bottom of the sea and providing transport between submarine hydrocarbon production wells, in particular oil and gas, and an installation surface, such as the underwater pipe 2 shown in Figures 1 and 2.

The underwater pipe 2 shown in these figures is typically assembled on the ground in several pipe sections of unit length 4 of the order of 10 to 100 m depending on the holding capacity of the laying system. We also speak of "joints", in particular "double seals" for two unit elements of pipe assembled together, "triple joints" for three unit elements of pipe assembled together, "quadruple joints" for four unit elements of pipe assembled between them, etc. For the rest of the description, the term "quad-joint" will be used generically to designate a unitary driving element 4. During installation, these quad-joints 4 are connected to each other on board the ship and at the as they are laid at sea.

Furthermore, the underwater pipe 2 is a coaxial pipe of the "Pipe in Pipe" (or "PIP") type, that is to say that each quad-joint (or unit pipe element) 4 comprises a pipe internal steel 6 for transporting hydrocarbons from production wells and an outer steel tube 8, coaxial with the inner tube and called "outer shell", which is in direct contact with the surrounding water.

In known manner, the annular space 10 formed between the inner tube 6 and the outer tube 8 of each quad-joint 4 can be filled with an insulating material 12 (or be emptied of any gas) so as to form a layer of thermal insulation limiting the heat loss to the environment. At each of these ends, the inner tube 6 can be secured to the outer tube 8 by means of connecting pieces 14 which close the annular space 10.

According to the invention, it is intended to monitor the mechanical behavior of such an underwater pipe 2, that is to say to monitor the mechanical stresses to which it is subjected when it is laid on the seabed. and during the network production phase (that is, when it is in service).

The monitoring method according to the invention provides two main steps: a so-called calibration step during which the mechanical signature of each quad-joint of the underwater pipe is obtained; and a monitoring step itself during which, from the mechanical signature of each quad-joint and measurements made by optical fiber sensors during the laying of the underwater pipe and / or during the phase of production of the network, the stresses undergone by the underwater conduct are determined.

Calibration step

This mechanical calibration step consists in testing each quad-joint of the underwater pipe on the ground in order to obtain its mechanical signature. This step is described in detail in the French patent application filed February 2, 2016 by the Applicant with the title "Method of monitoring the thermomechanical behavior of an underwater pipe for transporting fluids under pressure". For this purpose, each quad-joint undergoes in the laboratory or in the factory a series of different mechanical stresses of predetermined directions and intensities and the deformations undergone by the quad-joint during this test are measured by optical fiber sensors.

More specifically, the quad-joint 4 is equipped with measurement cables 16 provided with fiber optic sensors, each fiber optic sensor being intended to measure a single deformation parameter of the quad-joint.

The measurement cables can be positioned on the inner tube 6 of the quad-joint 4 (they are for example glued on the outer surface thereof) and extend longitudinally between the two ends of the quad-joint as described in the application. French patent application filed February 2, 2016 by the Applicant with the title "Method of monitoring the thermomechanical behavior of an underwater pipe for transporting fluids under pressure".

The optical fibers fitted to the measurement cables are monomode fibers using Brillouin backscattering for sensors intended to measure the mechanical stresses experienced by the quad-joint.

In the example described below, the mechanical stresses applied to the quad-joints of the underwater pipe are as follows: torsion stress (around the longitudinal axis 16 of the quad-joints), flexural stress (in a transverse YZ plane to the quad-joints), tensile / compressive stress (in the direction of the longitudinal axis of the quad-joints) and internal pressure loading to the quad-joints.

These stresses are all defined by a direction and intensity of application that are predetermined. Thus, as shown in FIG. 2, the torsional stress applied to the quad-joint 4 consists in the creation of a torsional moment (represented Mtx in FIG. 3) whose direction of application is along the axis X of an orthonormal reference (Ο, X, Y, Z) specific to the quad-joint. Similarly, the bending stress applied to the quad-joint consists in the creation of two bending moments (represented Mfy and Mfz in FIG. 3) whose directions of application are respectively along the Y and Z axes of the reference (Ο, X , Y, Z). The tensile / compressive stress applied to the quad-joint (shown in FIG. 3 by Nx) has an application direction along the X axis. Finally, the pressure bias is represented by P in FIG. 3 and its direction of rotation. application is radial to driving.

During the test, the different stresses detailed above are applied to each quad-joint and an optoelectronic OTDR measuring device (Optical Time Domain Reflectometer) commonly used to characterize optical fibers makes it possible to measure the deformation values, noted ε, experienced by the quad-joint.

Thus, thanks to the measurements ει to ε5 obtained by the optical fiber sensors, it is possible to obtain, in the case of elastic materials, the stiffness matrix Ω which satisfies the following equation (1):

(1) From the deformation values ει to Es measured by the optical fiber sensors and intensity values Mtx, Mfy, Mfz, Nx of the different mechanical stresses applied to the quad-joint during the calibration step, it is then possible to determine all the parameters gar, al2, a55 of the stiffness matrix Ω specific to the quad. Since the optical fibers traverse different paths on the surface of the inner tube of the quad-joint, this matrix is invertible. During mechanical calibration tests, the stresses must be limited in the elastic range of the constituent materials of the quad-joint.

This stiffness matrix Ω constitutes a mechanical signature that is specific to each quad-joint intended to constitute the underwater pipe. In fact, the parameters and the tolerances of the constituent materials (concentricity and thickness of the tubes, stiffness of the interannular insulation and spacers, characteristics of the welded joints, etc.) are different for each quad-joint, so that the matrix of stiffness varies from one quad-joint to another.

The mechanical signatures are in the form of digital files which are stored on board a memory of a computer system equipping the laying ship and / or the surface installation for later use for monitoring the mechanical behavior of the ship. underwater conduct during the actual monitoring step described below.

Acoustic calibration step

This acoustic calibration step takes place under the same operating conditions as the mechanical calibration phase. Each quad-joint equipped with a fiber-optic sensor using Rayleigh backscattering is grounded (ie, laboratory or factory) at different acoustic demands.

These acoustic solicitations comprise the emission of noise (in a useful frequency spectrum typically between 0 and 50 kHz) in different configurations: noise emission inside the quad-seal filled with air via a bar connecting the two ends of the quad-joint, emission of noise inside the quad-seal filled with water via a bar connecting its two ends, and emission of noise outside the quad-joint filled beforehand with air and water according to a calibrated protocol for each quad-joint (for example by means of a system emitting a calibrated sound vibration). At each noise emission during this calibration step, the optical time domain reflectometer (OTDR) used to characterize the fiber optic sensors makes it possible to measure the frequency variations of the quad-joint. These measurements are for example in a matrix form and constitute for each quad-joint its acoustic signature (that is to say, its basic reference).

As for mechanical signatures, the acoustic signatures of all quad-joints are in the form of digital files which are stored on board a memory of the computer system equipping the laying ship and / or the surface installation to be subsequently used for monitoring the behavior of the underwater pipe during the actual monitoring step described below.

Monitoring step

As shown in FIG. 3, during this step, the different quad-joints 4 intended to form the underwater pipe 2 are each equipped with a measurement cable 18 provided with at least one optical fiber sensor using the Brillouin and Rayleigh backscatter (for noise monitoring) and helically positioned around the inner pipe 6 of the quad-joint 4 (they are for example glued on the outer surface thereof).

More specifically, the measurement cable 18 of a quad-joint 4 of the underwater pipe forms a helix whose pitch p is constant and which forms an angle α with the longitudinal axis 16 of the underwater pipe, this angle a being preferably equal to 45 °.

In addition, for two adjacent 4-1, 4-2 quad-joints of the underwater pipe (see Fig. 3), the propellers formed by their respective measuring cables 18-1, 18-2 have respective pitches QzL Pi2 which are identical in absolute value but which are of opposite direction. In other words, the angles formed by these helices are inverse (for example one of these angles is + 45 ° while the other angle is -45 ° with respect to the longitudinal axis 16 of the underwater conduct).

This alternation of the pitch direction of the helices formed by the measuring cables of the adjacent quad-joints is found along the entire length of the underwater pipe. It starts from the assumption that at the scale of two consecutive quad-joints, the mechanical stresses Nx, Mfz, Mfy, and P are homogeneous in space. Thus, with a single optical fiber sensor, it is possible to calculate the deformation values Mtx, Mfy, Mfz, Nx, and P undergone by each quad-joint.

Also during the monitoring stage, the various quadrangements of the underwater pipe and equipped with their measuring cables are connected to each other on board the ship and as and when they are laid at sea as is known to those skilled in the art. This connection implies in particular that the measuring cables 18 are connected to each other, for example by passing them through holes 20 formed in the joining pieces 14 (see the magnifying glass of FIG. 1).

In addition, during the laying, the quad-joints 4 are indexed in rotation relative to each other, that is to say that the mark (Ο, X, Y, Z) specific to each quad-joint is located both in rotation and in translation in a more general reference through a geographic information system (GIS).

During the installation, and during the entire production phase of the network, measurements made by the optical time domain reflectometer (OTDR) of optical fiber sensors using Brillouin and Rayleigh backscatter are analyzed to determine the behavior of the pipe under -Marine to the deformations that she can undergo. This step of retrieval and analysis of the measurements taken by the reflectometer can be performed by the computer system equipped with appropriate software means and present on board the laying ship and / or the surface installation.

Indeed, starting from the equation (1) described above, and by inverting the matrix of stiffness Ω proper to each quad-joint forming the sub-matrix pipe, it is possible to calculate, thanks to a calculation software equipping the computer system, the deformation values Mtx, Mfy, Mfz, Nx, and P undergone by each quad-joint by applying equation (3) below:

(3)

In this way, it is possible to know at any time during the laying of the underwater pipe and throughout the production phase of the network the mechanical deformations undergone by each quad-seal forming the underwater pipe. In particular, it is thus possible to accurately identify where the underwater pipe occurs significant deformation of the pipe that can lead to significant damage or breakage of the pipe, or the formation of a plug inside the pipe can also cause a rupture of the pipe. This location is allowed thanks to the calibration phases which make it possible to know exactly the lengths and the location of the fibers deployed in the underwater pipe.

As for the monitoring of the noise variations inside the underwater pipe, it is obtained by comparing the measurements made during the whole production phase of the network by the optical time domain reflectometer (OTDR) of the optical fiber sensors using the Rayleigh backscatter with the acoustic signatures established for each quad-joint during the acoustic calibration step.

This step of recovery and analysis of the measurements taken by the time optical reflectometer can be performed by the computer system equipped with appropriate software means and present on board the laying ship and / or the surface installation.

Furthermore, it is also possible to monitor the temperature of the underwater pipe during the entire production phase of the network from thermal signatures established for each quad-joint as described in detail in the French patent application filed on February 2, 2016 by the Applicant with the title "Method of monitoring the thermomechanical behavior of an underwater conduit for transporting fluids under pressure".

Claims (8)

1. A method for monitoring the mechanical behavior of an underwater pipe for transporting fluids under pressure, the underwater pipe (2) being made by assembling a plurality of unitary pipe elements (4) end-to-end end, the method comprising: a mechanical calibration step consisting of at least one measuring cable provided with at least one optical fiber sensor using at least Brillouin backscattering and positioned over the entire length of the unitary conductor elements, to measure the deformations undergone by each unitary conductor element when subjected to different mechanical stresses of predetermined directions and intensities on the ground and, from these deformation measurements, to establish the mechanical signature each unit driving element; and a monitoring step consisting of a measuring cable (18) provided with at least one optical fiber sensor using Brillouin backscattering and positioned in a constant pitch (p) helix on each unit element of driving with alternating pitch direction of the helix for two adjacent unitary conductor elements, recovering variations of the optical signal injected into said fiber optic sensors when the line is in use, and from these signal variations optical and mechanical signature of each unit driving element, to determine the deformations undergone by each unit driving element. The method according to claim 1, wherein the helix formed by the optical fiber sensor of each unit pipe element forms an angle (a) with a longitudinal axis (16) of the underwater pipe of 45 ° by value. absolute. 3. Method according to one of claims 1 and 2, wherein the mechanical signature of a unitary driving element comprises a stiffness matrix whose values correspond to the deformation values undergone by the unitary driving element when it has has been subjected to the ground at the different mechanical stresses, the determination of the deformations undergone by each unitary driving element comprising the inversion of the stiffness matrix associated with the mechanical signature of each unitary element of pipe. 4. Method according to any one of claims 1 to 3, wherein the mechanical stresses of predetermined directions and amplitudes applied on the ground to each unit of pipe element comprise one or more of the following stresses: torsional stress, tensile stress / compression, flexural stress, and pressure stress. The method of any one of claims 1 to 4, wherein the optical fiber sensors of the measurement cables using Brillouin backscattering are monomode fibers. 6. A method according to any one of claims 1 to 5, wherein each unit pipe element (4) comprises an inner tube (6) mounted inside an outer tube (8) coaxially with the ci, the measuring cables with optical fiber sensors being positioned on the inner tube of said unitary pipe element. The method of any one of claims 1 to 6, further comprising: an acoustic calibration step consisting of a measurement cable provided with the fiber optic sensor using at least Rayleigh backscatter and positioned along the entire length of the unitary conductor elements, to measure the frequency variations of each unitary conductor element when subjected to different acoustic stresses on the ground, and, from these variations of noise, to establish an acoustic signature each unit driving element; and a step of monitoring the noise variations within the underwater pipe by recovering the frequency variations of the optical signal injected into the fiber optic sensors when the pipe is in use, and from these variations of frequencies and the acoustic signature of each unitary driving element, to determine the noise variations within each unitary driving element. 9. Device for monitoring the mechanical behavior of an underwater conduit for transporting fluids under pressure, the underwater pipe (2) being made by assembling a plurality of unitary elements of pipes (4) end-to-end end, the device comprising, for each unitary pipe element, at least one measuring cable (18) provided with at least one optical fiber sensor using Brillouin backscattering and positioned in a constant pitch helix on said unitary driving element , two adjacent unitary conductor elements being provided with measuring cables whose optical fiber sensors are helically positioned with an alternation of the direction of the pitch of the helix.